Last week, a bus full of opposition organizers from Belgrade was stopped at a police checkpoint, roughly halfway along the five-hour trip to a rally in the western Serbian town of Uzice.

Two police officers told the democratic activists to prepare their paperwork. A tense silence filled the bus. At the previous rally in Cacak, the "opposition bus" was stopped and turned back just outside of town, although the organizers walked the rest of the way. This time, the bus was still hours from its destination. Would it be allowed through?

The two police officers carefully examined everyone's paperwork, then let the bus pass. The relief was palpable and an occasion for a brief on-board celebration on the road to change the regime of Yugoslav President Slobodan Milosevic.

But only a half-hour before the Uzice rally was scheduled to begin, local police told organizers it was illegal. Electricity to the public-address system was turned off. The opposition ignored the police, brought in a generator, and spoke to several thousand people.

Serbia's postwar opposition movement is pressing forward despite such obstacles at every turn. Police harassment, phone taps, intimidation, threats, arrests, "informative talks," and people being laid off are daily occurrences.

"When people in the West say, 'Why don't you get rid of Milosevic?' they act like it's just that easy. We're faced with situations which are not imaginable to the average Western European or American," says an organizer of the Democratic Party. "And the bombing actually set us back, because the regime used it as an excuse to crack down."

Yugoslavia's opposition has taken various forms over the past decade to challenge Mr. Milosevic. Much of the time its efforts have been thwarted by infighting and Milosevic's authoritarian tactics. Yet one high point for the opposition was in 1996-97, when Zajedno ("Together"), a coalition of independent media, students, and opposition leaders, led three months of powerful demonstrations in Belgrade.

The conflict in Kosovo, however, effectively shut down most opposition, beginning as early as 1998. Milosevic took the opportunity to consolidate power and tighten his control of the media, thus muzzling voices of dissent.

Now, in the wake of defeat in Kosovo, oppositionists are beginning to raise their voices again. Attention has centered around two groups: the Alliance for Change, a coalition of smaller opposition parties led by Vladan Batic that is organizing rallies outside of Belgrade, and the larger Serbian Renewal Movement, led by Vuk Draskovic, which has won the support of about 15 percent of Serbia's population.

Whatever group opposition members belong to, threats and intimidation go with the territory. "When we distributed petitions [at the Uzice rally] calling for the removal of Milosevic, they threatened to arrest us, but then backed down," says Nenad Simovic, a Christian Democrat representative in Uzice.

In Nis, Yugoslavia's third-largest city, activists from the Democratic Party collected 12,000 signatures in two days calling for Milosevic to resign.

"There were long lines to sign our petition," says a Democratic Party activist in Nis. "Then the police showed up to warn us. Later plainclothes men from state security told us to leave immediately."

The petitioners now operate discreetly in downtown Nis.

Yesterday in Belgrade, the Democratic Party held a petition drive in 24 locations asking for Milosevic to resign. "Officially we're allowed to collect signatures, but police showed up at every location to intimidate and harass our activists," says Ljiljana Lucic, vice president of the Democratic Party. Some of the people collecting signatures were called in to the police station for what is known here as "an informative talk."

But opposition organizers expect much worse, and recent events in the southern Serbian town of Leskovac serve as an example of what is yet to come. Leskovac became the fulcrum of antiregime activity on July 1, when Ivan Novkovic, an editor with TV Leskovac, aired a videotape of himself speaking to the camera during the halftime broadcast of the highly watched Yugoslavia-Germany basketball game.

Mr. Novkovic politely invited the citizens of Leskovac to "a meeting" to call for the resignation of local Socialist strongman and district leader Zivojin Stepanovic - a Milosevic crony.

Novkovic's hands trembled as he asked Mr. Stepanovic a series of rhetorical questions: "How do you feel when you fire people because they think differently than you? Were you initiating positive changes when you sent 40,000 people to war? Do you believe in God?"

Twenty thousand people showed up to Novkovic's "meeting," and he is now an opposition hero while he serves his 30 days in jail. His trial was swift and secret, and he'll have no job when he gets out. The other eight people who worked with him on that shift were placed on temporary leave.

In Belgrade, where offices of the Christian Democratic Party of Serbia serve as the headquarters of the Alliance for Change, mobile phones don't work, and other peculiarities have been noted.

"We know our offices are bugged," says Ms. Lucic. "That's just part of being the opposition."

Opposition leaders now speak of absolute sacrifices, thoughts born in extreme desperation. "If people in the opposition are scared of organizing, they're much more scared of the idea that Slobodan Milosevic will still be in power in the year 2000," says Slobodan Orlic, vice president of the Social Democrats.

"I lost my job because of politics. So what? We have to let go of fear. We expect to be intimidated and harassed, but they can't arrest all of Serbia," says Goran Svilanovic, president of the Citizen's Alliance, an opposition group.

"In Serbia, being involved in professional opposition politics entails enormous risks, but we have to press forward. We have no choice," says Zivojin Stjepic, of the Christian Democrats.

This week, opposition demonstrations are planned in Jagodina and Kraljevo.